heart–lung machine

heart–lung machine Operating on the human heart poses problems which inhibited surgery on the heart until the early 1950s. Manipulation of the heart, and opening of its cavities' interferes with its function and its ability to sustain the circulation. The heart–lung machine is a system which takes over the function of the heart and the lungs with sufficient safety to maintain life while the heart is stopped or opened to allow surgery on the coronary arteries or the heart valves, or to allow repair of congenital abnormalities.

While in theory it is only necessary to bypass the function of the heart, it soon became apparent that in practice it is simpler to bypass the function of both the heart and the lungs. The main components of a heart–lung machine are a pump (to provide the driving force to the blood in the arterial system), an oxygenator (for exchange of oxygen and carbon dioxide), and a heat exchanger (to allow control of temperature of the body). The connecting tubing and filter are other components of the heart–lung bypass circuit.

Venous blood is siphoned from the body via a tube in the right atrium of the heart, or via two tubes in the major veins which converge on the heart. It is pumped through the oxygenator and heat exchanger, and returned via a plastic tube into the arterial system of the body — usually at the upper portion of the ascending aorta (see blood circulation).

The design of pump which is in most common use today is the roller pump — a simple rotating arm carrying rollers which compress a loop of polymeric tubing against a solid surface. Speed of rotation of the roller-bearing arm is controlled to allow a pumping rate similar to that of the normal heart at rest (about 2.4 litres/min/m² body surface — or typically about 5 litres/min in an adult).

There are two main types of oxygenator in use at present. ‘Bubble oxygenators’ expose the passing blood to a stream of gaseous bubbles composed of 95% oxygen and 5% carbon dioxide. Gas exchange with the blood occurs on the surface of the bubbles and results in reasonably normal levels of oxygenation of the blood and maintains carbon dioxide in the normal physiological range. The bubble oxygenator has a sponge-like filter and reservoir to enable gaseous bubbles to be removed from the oxygenated blood before it is pumped back to the body.

Membrane oxygenators consist of a series of fine tubes which allow diffusion of oxygen and carbon dioxide between the blood flowing through them and the ventilating gas surrounding them (or vice versa).

The oxygenator also combines with a heat exchanger — a system of tubes through which the blood passes, surrounded by circulating water at controlled temperature. This allows the blood temperature to be maintained (counteracting the heat loss during the passage of blood through the heart–lung machine). It also allows deliberate cooling and subsequent rewarming of the blood, giving the surgeon the option of reducing, or even stopping, the circulation of the blood around the body for a period of time with safety, because the oxygen requirement of the body is reduced by hypothermia.

The connecting tubes, the oxygenator, and the pump tubing are all filled with a physiologically compatible fluid (priming fluid) prior to final connection with the circulation of the body. Avoidance of air bubbles in the heart–lung circuit is of vital importance. Exposure of blood to the foreign surfaces of the heart–lung machine initiates the natural clotting mechanisms of the body, and this must be inhibited by giving the drug heparin to the patient before allowing the circulation to be taken over by the heart–lung machine. Normal blood clotting is restored after the operation by the administration of protamine, which neutralizes the heparin.

The heart–lung machine has made virtually all the advances in cardiac surgery possible. With the function of the heart and lungs taken over temporarily by artificial means it is possible to stop ventilation of the lungs, and to stop the heart, and open the coronary arteries or the cavities of the heart for repair or replacement of the heart valves, or to undertake the correction of congenital abnormalities of the heart.

For periods of up to two or three hours (usually adequate for most surgery) the heart–lung machine is safe; beyond this time there is a risk of increasing damage to the red cells of the blood. Exposure of blood to the foreign surfaces of the artificial circuit initiates an inflammatory response throughout the body, and there is an impairment of function of many organs for a short period after surgery. Nevertheless, the heart–lung machine has become a safe and crucial component of virtually all surgery on the heart and on the major blood vessels around the heart.

D. J. Wheatley

Bibliography

Millner, R. and and Treasure, T. (1995). Explaining cardiac surgery: patient assessment and care. BMJ Publishing Group, London.